Forests of spruce trees and shrubs are taking over tundra in parts
of northern Canada, forcing out other species. New research examining
the last three centuries of arboreal history shows that this shift
can happen at a much faster speed than scientists originally thought.
The study from the University of Alberta appears in the March issue
of the Journal of Ecology.

“The conventional thinking on tree-line dynamics has been
that advances
are very slow because conditions are so harsh at these high latitudes
and elevations,” says UA biologist Ryan Danby. “But
what our data indicates is that there was an upslope surge of trees
in response to warmer temperatures. It’s like [the trees]
waited until conditions were just right and then decided to get
up and run, not just walk.”

Danby and UA’s David Hik reconstructed changes in the density
and elevation of tree-line forests in the southwestern Yukon for
the past 300 years. Using tree rings, they were able to date the
years of establishment and death of spruce trees and reconstruct
changes in treeline vegetation.

Danby and Hik found that a rapid change in response to climate
warming during the early to middle part of the 20th century was
observed at all locations. The tree line advanced considerably—as
much as 85 meters (280 feet) in elevation—on south-facing
slopes, where sunshine warmed the soil. Tree density increased
significantly—as much as 65%—on cooler, north-facing
slopes.

“The mechanism of change appears to be associated with occasional
years of extraordinarily high seed production, triggered by hot,
dry summers, followed by successive years of warm temperatures
favorable for seedling growth and survival,” says Danby.